If it were me, I would use a crock pot to reheat, not only because of the ease of usage but also because of its type of low and slow heating it will help you maintain moisture.
BUT....... if the grill is the way to go, I reiterate Demosthenes9's suggestion. I would double pan and I would spill some of your recovery fluid (apples juice or whatever between the pans so its like a double boiler effect, no not enough to make the inner pan float but whatever I could get between the two, then when you foil it will not only steam off a little into your inner tray but hopefully you not burn the meat on the bottom.
Again though, I suggest a crock pot. They were just made for this kind of use. Also I would add a little finishing sauce before bagging for the reefer. That is why pulled pork is always better the second day. it allows the finishing sauce to marry with the meat. At my house a 9 lb butt might only take a Tablespoon, at the most two when pulled. BUT the next day people will be talking about the wonderful flavors you have in your pork. You don't have to tell your secret why yours is better than the local BBQ stand.
Good luck and I hope it all turns out great!
BTW I prefer "JJ's finishing sauce" which there a recipe posted on the board to make. It lasts a long time because you don't need much! And it wakes up a pulled pork to excite the taste buds and buddettes!